The present invention generally relates to base stations and transmission power determining methods in mobile communications systems and, more particularly, to a base station and transmission power determining method which determine the transmission power in consideration of only neighboring cell interference margin for preparing for neighboring cell interference.
In mobile communications systems utilizing CDMA (Code Division Multiple Access) system, etc., signal-to-interference power ratio (SIR) satisfying predetermined reception quality (such as Bit Error Rate (BER) or Block Error Rate (BLER)) at a base station is measured. A high speed transmission power control (TPC: Transport Format Combination Indicator) based on the measurement can be applied to uplink and downlink to minimize the transmission power with respect to a required reception quality and therefore increase system capacity.
In the uplink, especially, when plural mobile stations transmit with the same power, their transmission losses become larger as mobile stations are located farther from the base station. As a result, the received signal from a mobile station located far from the base station (that is close to the cell edge) is masked by the received signal from a mobile station close to the base station at a base station receiver input. In order to solve this problem, the high speed TPC is needed to control the transmission powers of mobile stations so that the SIR from each mobile station is constant at the base station reception.
The high speed TPC based on the SIR measurement comprises an inner loop and an outer loop. In the inner loop, the SIRs of signals after Rake-synthesizing each slot are measured, and the transmission power is controlled so that the measured SIR becomes equal to a target SIR. In order to control the transmission power, two value TPC commands are generated and transmitted using one of a pair of links (for example, using down link when controlling uplink).
It is known that reception power distribution when performing the high speed TPC is approximate to a logarithmic normal distribution. In order to realize highly accurate TPC, precise SIR measurement is required. A prior method was proposed, in which each path SIR is measured and added to obtain equivalent SIR after Rake-synthesis instead of direct measurement of SIR after Rake-synthesis. In this method, more precise SIR measurement can be performed compared with the direct measurement of SIR after Rake-synthesis, because the influence by the channel estimation error can be reduced.
In the inner loop, the revision of the transmission power is done for each slot (for example, 0.667 ms). Large step sizes for revising the transmission power can follow sudden and drastic changes in transmission path. However, too large a step results in a large distribution or deviation in reception power compared with a constant status and degrades characteristics. As a result, it is reported that a step size of 1 dB provides the best characteristics.
On the other hand, the same target SIR may provide different reception qualities (BLER or BER), depending on transmission environment such as the number of transmission paths and a moving speed (maximum Doppler frequency) of the mobile station, difference in SIR measurement methods and interference variation, etc.
Accordingly, it is possible by the outer loop to measure reception quality over a long period and correct (set a correcting margin) a target SIR with a slow period based on the measured reception quality. However, in high quality and high speed data transmission technology, in order to improve the following characteristics against the outer loop transmission environmental variation, another method can be utilized, in which binary or two-valued determination data after error correction and decoding are re-channel coded instead of correcting the target SIR with BLER, and the BER of a provisional determination data train after Rake-synthesis is obtained using the data train after interleaving as reference data. The target SIR is corrected so that the measured value becomes equal to the target BER value.
In line switching situations, traffic varies only per session, and interference variation is not so drastic. On the other hand, in packet communication, traffic does change per packet, and the interference variation is significantly greater compared with line switching. It is very difficult to follow the significantly varying interference in packet communications. In the prior art, in order to deal with this significant interference variation, a target reception power for the next time unit is controlled based on the present measured total interference including interference margin.
However, there is a problem in that adding a large interference margin increases transmission power and decreases system throughput.
The inventors of the present invention found that when using a reservation type access system, a base station can completely forecast or predict interference within its own cell for the next time unit based on allocation information (because all transmissions are allocated by the base station). And therefore margin for dealing with the interference within its own cell is not needed. The reservation type access system is a system in which a base station receives a reservation signal from a mobile station before packet transmission, and the base station allocates a radio resource required for transmitting packets.